Colour display tubes are used, for example, in television receivers and monitors.
A quadrilateral supporting frame is a frame having two pairs of opposed sides.
Colour display tubes of the type described above are known. The shadow mask contains a large number of apertures to allow passage of electron beams and extends approximately parallel to the display screen. After the electron beams have passed through the apertures in the shadow mask they impinge on the display screen. The display screen contains a phosphor pattern. The location where electron beams impinge on the display screen is governed, inter alia, by the position of the apertures of the shadow mask relative to the phosphor pattern of the display screen. As approximately 80% of the electrons generated by the electron gun impinge on the shadow mask, the temperature of the shadow mask increases in operation. This causes the shadow mask to expand. In general, the temperature increase at the edge is smaller than at the center of the shadow mask. This difference in temperature causes the shadow mask to bulge. As a result thereof, displacements of the electron beams relative to the phosphor pattern may occur, which displacements will hereinafter be referred to as "landing displacements". Landing displacements taking place when the shadow mask is uniformly irradiated are referred to as "overall doming". Landing displacements also occur when the ambient temperature changes; this phenomenon is termed "ambient doming". By using suspension means, a supporting frame can be suspended, in known manner, in front of the display screen in such a way that the distance between the supporting frame and the display screen is temperature-dependent. Suspension means which can suitably be used for this purpose may comprise, for example, bimetal springs or another suspension construction which move or moves the supporting frame toward the display screen when the temperature increases. In this manner, "overall doming" can be reduced.
A reduction of the landing displacements has a favourable effect on picture quality.